dc get a mixture - in their stocking there's always a new vest, pair of pants, pair of socks and christmas tshirt - then these things are conveniently to hand and exciting to put on for christmas day when trying to chivvy them to get dressed rather than stay in their pjs when we need to be getting dressed.
They probably wear it with trousers that they already have - although I will have made sure that the jeans or whatever they particularly like wearing at the time are clean and don't have holes in!
Typically the underwear from the stocking will have come from multipacks - so the rest of those will turn up under the tree. Might have to be careful with that this year - ds1 is now 7 and old enough to start to care about reading packages and spotting that the pack of 7 pants only has 6 in it or whatever!
Boxing day we often have guests over so I'll have made sure there's another nice new tshirt or shirt to be opened - just makes life easier when in the morning again as they like to put clean new stuff on. Although it does mean that they often end up wearing tshirts that are on the big side (ds's birthday is in may so I will often end up buying the size he's going to be). It's all family though for both days so I'm happy to see them in a nice tshirt - then they are comfortable but still looking reasonably respectable (I'm rubbish at ironing and never bother with it).
If we've been to a wedding in the year or something similar where they have had nice outfits, I might try to co-erce them into wearing them if we have guests or go somewhere nice, but dh is the eternal scruff bucket and hates dressing up for anything other than a wedding or an interview (and has been known to dress down just to point this out
) (well he doesn't like dressing up for the wedding or the interview either but he accepts there are times that he needs to look smart - just never when he's out with us!)
I can see however that if you have little girls that you might not be able to wrap up a nice dress and hope for it to remain crease free when stuffed in a stocking and then ripped open and thrown into the present pile to bring in to show parents quite as easily as a tshirt survives it all!